Field of the Invention
The present invention relates to a surface emitting laser and an optical coherence tomography apparatus including the surface emitting laser.
Description of the Related Art
Vertical cavity surface emitting lasers (VCSELs) are an example of surface emitting lasers. VCSELs include an active layer and two reflectors that sandwich the active layer from above and below to form a resonator in a direction perpendicular to the surface of a substrate. VCSELs emit a laser beam in the direction perpendicular to the surface of the substrate. Also, wavelength tunable VCSELs, with which the wavelength of emitted light can be varied, are known. In an example of such a VCSEL, a gap portion is provided between an upper reflector and an active layer of the VCSEL, and the wavelength of emitted light can be varied by varying the cavity length by moving the upper reflector in a direction of the optical path of the laser beam. Surface emitting lasers with which the wavelength of the emitted light can be varied are hereinafter referred to as wavelength tunable VCSELs.
It is known that wavelength tunable VCSELs are suitable for use as light sources of optical coherence tomography (OCT) apparatuses. In the case where a wavelength tunable VCSEL is used as a light source of an OCT apparatus, the tunable wavelength range of the wavelength tunable VCSEL may be increased to increase the axial resolution of the OCT apparatus. The tunable wavelength range of the wavelength tunable VCSEL may be increased by, for example, increasing the reflectances of reflectors arranged above and below the active layer over a wide reflection range.
A wavelength tunable VCSEL in which a pair of distributed Bragg reflectors (DBRs) are arranged above and below an active layer is disclosed in IEEE Journal on Selected Topics in Quantum Electronics, Vol. 6, No. 6, November 2000 (hereinafter referred to as Non-Patent Document 1).
To achieve a high reflectance for laser oscillation, DBRs having a multilayer structure including dielectric or semiconductor layers are generally used as reflectors. A general DBR is formed by alternately stacking two types of layers having different refractive indices such that the optical thickness of the layers is ¼ of the center wavelength of the wavelength range of the laser beam. Here, the optical thickness of a layer is the product of the thickness of the layer and the refractive index of the material of the layer. The reflectance and reflection range of a DBR are determined by the difference in refractive index between the two types of layers and the number of layers that are stacked. As the difference in refractive index between the two types of layers and the number of layers that are stacked increase, the reflection range and the reflectance increase (see FIGS. 9A and 9B). When the wavelength range in which the reflectance is high is increased, the reflectance itself is also increased (see FIGS. 9A and 9B). When the reflectance of the reflector at the light emission side of a laser is too high, light cannot be emitted. When the reflectance is too low, the threshold of laser oscillation increases. Therefore, the reflectance of the reflector at the light emission side is set to an appropriate value.
More specifically, when the number of layers included in a DBR is increased to obtain a high reflectance over a wide range, there is a risk that the reflectance will be too high and it will be difficult for light to be emitted out of the laser. In contrast, when the reflectance is low, the amount of light that leaves the resonator increases, and it becomes difficult to achieve laser oscillation. Thus, the light emission efficiency may be reduced when the reflectance is too high, and the threshold current for emitting a laser beam may be increased when the reflectance is too low.
In consideration of the above-described circumstances, the present invention provides a surface emitting laser including a reflector having reflectance characteristics with which light emission efficiency is not reduced and a threshold current is not increased over a wide wavelength range.